Page 59 - Chemical engineering design

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The component flows in a stream (or the quantities in a batch) are completely defined
by any of the following: CHEMICAL ENGINEERING
1. Specifying the flow (or quantity) of each component.
2. Specifying the total flow (or quantity) and the composition.
3. Specifying the flow (or quantity) of one component and the composition.

Example 2.6
The feed stream to a reactor contains: ethylene 16 per cent, oxygen 9 per cent, nitrogen 31
per cent, and hydrogen chloride. If the ethylene flow is 5000 kg/h, calculate the individual
component flows and the total stream flow. All percentages are by weight.

Solution

Percentage HCl D 100 16 C 9 C 31 D 44
5000
Percentage ethylene D ð 100 D 16
total
100
hence total flow D 5000 ð D 31,250 kg/h
16
9
so, oxygen flow D ð 31,250 D 2813 kg/h
100
31
nitrogen D 31,250 ð D 9687 kg/h
100
44
hydrogen chloride D 31,250 ð D 13,750 kg/h
100
General rule: the ratio of the flow of any component to the flow of any other component
is the same as the ratio of the compositions of the two components.
The flow of any component in Example 2.6 could have been calculated directly from
the ratio of the percentage to that of ethylene, and the ethylene flow.
44
Flow of hydrogen chloride D ð 5000 D 13,750 kg/h
16

2.10. GENERAL ALGEBRAIC METHOD
Simple material-balance problems involving only a few streams and with a few unknowns
can usually be solved by simple direct methods. The relationship between the unknown
quantities and the information given can usually be clearly seen. For more complex
problems, and for problems with several processing steps, a more formal algebraic
approach can be used. The procedure is involved, and often tedious if the calculations
have to be done manually, but should result in a solution to even the most intractable
problems, providing sufficient information is known.